The most important step in your GRE preparation is to first strengthen all of your core concepts and fundamentals in the four categories of GRE Quantitative section: Arithmetic, Algebra, Geometry, and Data Analysis. This is the most time consuming step of the preparation, and will require you to learn all the core concepts and strengthen your understanding by practicing with official GRE questions.
Start with the complete GRE Course and complete all the concept lessons and the associated practice problems. The course consists of 15 concept lessons and you will be required to complete about 450 practice questions as part of the homework. One-third of the questions are selected from the Official Guide to the GRE General Test 3rd Edition, and I would recommend you to buy this guide.